In the words of the great Douglas Adams, "Time is an illusion. Lunchtime doubly so." And even though time may be a construction created by humans to make sense of our universe, we all need to be able to measure it accurately to project our own futures. That's what calendars are for. In practice, ours are so that we don't agree to go to a Star Trek convention the same day as the Zelda concert. In addition to the more traditional wall calendars and desk calendars in the day-by-day format, we also have perpetual calendars, some of which, for instance might track the progress of a young Padawan so that you can write down the day you first said, "The Force is strong with this one."